1. Field of the Invention
The present invention relates to an original reading device that feeds an original in a sub-scanning direction on a contact glass and has a line image sensor that reads a plurality of lines of an image in a main-scanning direction at a reading position on the contact glass, and to an image forming apparatus having the original reading device.
2. Description of the Related Art
In a conventional image reading device of a sheet-scanning type (or a sheet-through type) that feeds an original in a sub-scanning direction on a contact glass and has a line image sensor that reads a single line or a plurality of lines of an image in a main-scanning direction at a reading position on the contact glass, an image of dust, if attached to the contact glass, is read, and a vertical black line image appears on a read image when the dust is not removed.
When such a vertical black line image appears on a read image, a user has to clean the contact glass and have the image reading device to read the image again.
In a technique disclosed in Japanese Patent Application Laid-Open No. 2006-229719, which is made to cope with the problem, dust is detected on a reading surface in a sheet-through reading (automatic document feeding, skimming), by determining whether a line extending in a sub-scanning direction which remains in its position in a main scanning pixel or in its data size is present. Determination is possible because any line on an original, no matter how straight the line is, changes its position in a main scanning pixel or its data size due to skew caused at the time of feeding the original.
In this conventional example, dust on a reading surface is detected when it is determined that a line extending in a sub-scanning direction which remains in its position in a main scanning pixel or in its data size is present.
This detection method requires detection over a number of lines in a sub-scanning direction. Therefore, the larger memory capacity for storing therein read image data necessary for the detection is needed, and the cost of the apparatus increases.